It previously returned the cookie directly, which made it impossible
to distinguish between a NULL cookie and the function not having
anything to dequeue. This lead to some code setting a cookie that was
not actually used.
Return the dequeue status as a boolean and provide the cookie with an
optionally handed in pointer instead and adjust all users.
Change-Id: Iaac1726ac4bc7ae42bb96b8f0915852b6def5822
Reviewed-on: https://review.haiku-os.org/c/haiku/+/1814
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>
Reviewed-by: waddlesplash <waddlesplash@gmail.com>
9a2911ca8c